Here are a few contenders:
Based on sheer size (volume):
* Armillaria ostoyae (Honey Mushroom): This fungus, found in Oregon's Malheur National Forest, covers an area of 2,385 acres (965 hectares) and is estimated to be 2,400 to 8,650 years old. While it has multiple fruiting bodies, it is considered a single organism due to its interconnected network of mycelia.
* Pando (Quaking Aspen Clone): This clonal colony of quaking aspen trees in Utah covers 106 acres (43 hectares) and is estimated to be 80,000 years old. While each tree is genetically identical and connected by a single root system, some argue it's not a single organism but a collection of interconnected individuals.
Based on biomass (total weight):
* Posidonia oceanica (Neptune Grass): This seagrass meadow in the Mediterranean Sea covers an area of over 150,000 acres (60,000 hectares) and is estimated to be 100,000 years old. It's a single organism due to its interconnected rhizome system, making it arguably the largest organism on Earth by biomass.
Ultimately, the answer depends on your definition of "single organism" and "largest".
The debate continues!
